Systemic lupus erythematosus (SLE) is a systemic autoimmune disease with protean clinical presentation and variable outcome; and glomerulonephritis (GN) is a serious manifestation of SLE. SLE patients and lupus prone mice exhibit abnormalities in B cell and T cell tolerance and their responsive state. They produce autoantibody (autoAb) response to multiple self antigens (Ag) and have immune complex (IQ deposits in tissues. The CD4+CD25+ regulatory T cells normally prevent organ specific autoimmune disease occurrence. Herein we investigate the role of the regulatory T cells in SLE by studying the effect of thymectomized on day 3 (d3tx) in lupus prone mice. The d3tx lupus prone NZM2328 mice exhibited earlier autoAb response, accumulated more glomerular IC, and developed accelerated acute GN. Remarkably, severe acute GN occurred in - 90% of male NZM2328 mice that are normally more resistant to lupus GN. These results have led to the following hypotheses. First, regulatory T cells can negatively influence development of SLE. Second, acute lupus GN is a T cellmediated autoimmune disease that targets the renal glomerulus. The CD4+CD25effector T cells participate in renal glomerular injury by targeting either renal Ag or Ag provided by the glomerular 1C. Thus, in SLE, autoreactive T cells: 1) drive an autoAb response, and 2) directly elicit acute GN. Third, acute GN is a checkpoint in lupus GN, from which the male mice regress and the female mice progress to chronic GN. Thus male NZM2328 mice may have less effective or more regulatory T cell function. The model will also permit the study on factors responsible for the progression and regression of lupus GN. We will investigate these hypotheses in Aim I of our proposal. A very different story emerged from the study on the lupus prone female SNF1 mice. Following d3tx, an accelerated autoAb response was also noted but this was associated with significant reduction in fatal GN. Analysis of their serum and glomerular autoAb isotypes revealed an autoimmune response with a strong Th2-bias. In Aim 2, we will test the hypothesis that d3tx of the SNF1 mice retains the nonpathogenic neonatal Th2 responsiveness. At the same time, they had a reduced Ag specific Th1 response that is required for pathogenic autoAb production and for lupus GN. Finally, we will seek an explanation for the different responses to d3tx between the two lupus prone mice. Accordingly, we will determine the outcome of CD4+CD25+ T cell depletion by methods other than d3tx in SNF1 mice.